A well-known and particularly annoying artifact in flash images is the so-called “red-eye effect”, where people's eyes appear red instead of their natural color. Therein, the red color stems from a reflection of the flash light on the blood-rich retina. The effect is particularly pronounced when the flash light is arranged near the optical axis of the camera lens, which is particularly the case when deploying small-sized cameras, such as for instance compact cameras or cameras integrated into mobile appliances (e.g. mobile phones).
The red-eye effect may be combated in a plurality of ways.
A particularly simple way is to move the flash light away from the optical axis of the camera lens, so that the camera lens does no longer receive the direct reflection of the flash light from the retina. However, this approach is in general not feasible for compact cameras or cameras that are integrated into mobile appliances due to size constraints.
According to a further approach, described in U.S. Pat. No. 4,285,588, a pre-flash is used that causes the pupil to close before the actual image is captured. The time between the pre-flash and the actual capturing of the image has to be chosen large enough to allow the pupil to close. In U.S. Pat. No. 4,285,588, a time delay of around 600 ms is preferred. This however increases the latency of image capture, and furthermore is not suited to entirely remove the red-eye effect, since the pre-flash will not cause the pupil to close completely.
Removal of the red-eye effect may furthermore be accomplished by an algorithm processing a captured image and attempting to identify and remove red eyes contained therein, for instance based on geometrical constraints prescribed by the form of the eye. However, both the rate of false and missed detections of red eyes is generally quite high.
U.S. patent application publication no. 2006/0008171 describes a further approach for red-eye removal, which relies on taking an image pair, comprising an image taken without flash and a subsequent image taken with flash. Therein, both images are taken in short succession, for instance within 1/30 of a second. The difference in red chrominance between the no-flash image and the flash image is determined, and, based on a threshold value, it is determined which regions may form potential red eyes. Subsequently, the red-eye regions are removed. However, the performance of this approach critically depends on the choice of the threshold value. Furthermore, performance further significantly degrades if the no-flash image is too dark.